The present invention relates to microelectronic connection components and more specifically relates to microelectronic assemblies incorporating soldered connections and to components incorporating pads for soldering.
Soldered connections are used throughout the electronics industry to connect electronic components. Where the components to be connected include dielectric elements such as a rigid circuit panel or a flexible dielectric circuit panel with conductive metallic traces, the traces may be provided with enlarged regions, commonly referred to as "lands" or "pads". The traces may extend along a surface of the dielectric element. A further dielectric element, commonly referred to as a "solder mask" or "coverlay" may be provided. The solder mask layer may be applied by laminating a preformed dielectric sheet to the surface of the dielectric element, or by forming the dielectric sheet from a curable liquid on the surface of the dielectric element. The solder mask has holes at spacings corresponding to the spacings of the pads. The solder mask coverlay closely overlies the trace-bearing surface of the panel and closely overlies the metallic traces, leaving all or part of each pad exposed at the corresponding hole in the solder mask. A mass of solder may be deposited on each pad, either by exposing the assembly to a liquid solder, as in a wave soldering or dip soldering process or, more typically, by applying solder performs commonly referred to as "solder balls" onto the pads and heating the assembly to melt the solder. The molten solder wets the metal of the pads and forms a strong bond to the pads. The solder mask layer, which is not wettable by the solder confines the solder on the pads. In the absence of the solder mask layer, the molten solder could wet the metal in the traces extending away from the pads and hence could flow outwardly, along the traces. This would provide solder in undesired locations and displace the solder mass from its desired location, centered on the pad. Moreover the undesired solder flow can remove solder from the pads where it is required for forming the joints. The solder mask prevents this undesired flow.
After application of the solder masses, the component has solder masses protruding from the surface. A component such as a semiconductor chip package having an array of solder masses on a surface in a grid-like pattern is sometimes referred to as a "ball-grid array" element. The use of ball-grid arrays in packages for microelectronic devices such as semiconductor chips is described for example, in the article "TBGA Package Technology," IEEE Transactions on Components, Packaging and Manufacturing Technology, Part B, Vol. 17, No. 4, VP 564-568 by Andros and Hammer and in "Ball Grid Array Technology," Lau, J. H. ed, pp. 460-464. As described, for example, in commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266, a microelectronic component such as a semiconductor chip may include a set of pads in the form of terminals which may be mounted on a dielectric layer such as a flexible sheet. The pads or terminals may be connected to contacts on the chip by flexible leads and may be supported above the surface of the chip by a compliant layer such as an elastomer interposed between the terminals and the chip, typically between the dielectric layer and the chip. Masses of solder may be provided on the pads or terminals for connecting the assembly to a circuit board or other substrate having corresponding pads.
The component can be engaged with another component having a corresponding set of pads and, typically, also having similar solder mask. After engaging the protruding solder masses with the pads of the other component, the solder masses may be heated again to melt all or part of each solder mask and bond the solder masses to the pads of the other component. The resulting solder columns interconnect pads on both components with one another electronically and also form a mechanical connection between the components. In a variant of this process, each solder ball may include a core, typically formed from a conductive metal such as copper or nickel which does not melt at the temperatures used to melt the solder. Such a core is commonly referred to as a "solid core". A "solid" core in this context may have one or more interior voids, or else may be entirely free of voids. The resulting solder joint includes the solid core embedded within an outer layer of solder. Alternatively, the protruding solder elements may be engaged with a small socket having holes and metallic resilient elements designed to accept and engage the solder elements in the holes. Sockets of this type are disclosed in certain embodiments of International Patent Publication WO 95/34106. Other sockets which can engage protruding solder or other elements are described in International Patent Publication WO 97/44859.
One particularly useful approach described in copending commonly assigned in U.S. patent Application Ser. No. 411,472 filed Mar. 28, 1995 incorporates solid core solder assemblies in components with having pads supported on a compliant layer. In certain structures according to the '472 application, the compliant layer allows the pads and hence the solid core solder assemblies to move when the component with the assemblies thereon is juxtaposed with another component. This allows the solid core solder assemblies to engage the pads on the opposite component despite some minor deviations from perfect planarity of the pads on the components. Moreover, the compliant layer in the assembly mechanically decouples the pads from movement of the underlying component caused by thermal expansion and contraction and thus reduces the mechanical stresses, such as fatigue on the solder assemblies. The mechanical decoupling also facilitates use of the component with a socket adapted to receive the solid core solder balls.
Some typical configurations for pads or lands are described for example in American National Standard ANSI/IPC-D-249, Design Standard for Flexible Single and Double Sided Printed Boards; in American National Standard ANSI/IPCD-275 of September 1991 entitled Design Standard for Rigid Printed Boards and Rigid Printed Board Assemblies, pp.62-68; in Design Guidelines for Surface Mount and Fine--Pitch Technology, 2nd edition, 1996 by Vernon Solberg, pp.142-143; and in Fjelstad, An Engineer's Guide to Flexible Circuit Technology (ElectroChemical Publications Limited 1997; ISBN 0901150347), pp.148-149.
Many common pads are round, circular bodies of conductive material. However, other shapes, such as hollow circles or squares of conductive material have been employed. Also, round and square lands with branches or ribs extending from them have been employed as illustrated in Standard ANSI/IPC-D-350, Revision D, July 1992 and in Natarajan et al., U.S. Pat. No. 5,519,580. Still other solder land configurations are disclosed in IBM Technical Disclosure Bulletin, Vol. 40, No. 6, pp.199-200 entitled "Ball Grid Array Solder Ball on Ball Grid Array Dimple Pad" (June 1997); in U.S. Pat. No. 5,400,220 and in U.S. Pat. No. 5,133,495.
Despite all of these efforts in the art however, further needs for improvement in microelectronic assemblies incorporating solder joints, and in method components for making such joints, remain.